The present invention is in a method for preparing 1,2-dichloroethane from ethylene and chlorine gas.
It is known to prepare 1,2-dichloroethane (hereinafter alos referred to as ethylene dichloride) by reacting ethylene with chlorine in liquid ethylene dichloride in the presence of Lewis acid catalysts. The known large-scale processes operate continuously, at standard pressure or slightly elevated pressure, below or at the boiling temperature of ethylene dichloride. The object in this case is to achieve high volume-time yields and product yields along with a very complete conversion of the ethylene, and, by removing the considerable heat of reaction, maintaining a constant temperature in the reactor (cf. German Pat. No. 2,427,045). To accomplish this object, different approaches are used.
For example, the reaction heat is removed by a cooling system installed within the reactor, or the performance of the chlorination reaction on the one hand and the removal of the reaction heat on the other are carried out in separate apparatus, heat being removed from the product by temperature reduction in coolers disposed outside of the reactor. The effectiveness of either cooling system is gradually impaired by incrustation with complex compounds and poorly soluble products, so that difficult cleaning operations are required at intervals of time to sustain the performance of the apparatus.
German Offenlegungsschrift No. 29 35 885 describes a combination of expedient chemical and process engineering measures by which, among other things, the main reaction of the addition of chlorine onto ethylene is performed at pressures of 0.3 to 1.3 bar and temperatures of 50.degree. to 90.degree. C., and at least half of the chlorine is used in solution in the cooled reaction product and the rest of the chlorine is added in gaseous and/or liquid form.
More recent methods operate at temperatures of over 100.degree. C. in order to have a sufficient temperature level for utilization of the reaction heat of about one ton of steam per ton of ethylene dichloride. The heat removal is performed again by direct cooling of the reactor contents or by the evaporative cooling of ethylene dichloride, the regeneration of the reaction heat being performed as a rule by cooling or condensing with condensate with steam recovery, or other units of the process are heated directly with the vapors of ethylene dichloride that are produced.
Other differences lie in the feeding of the ethylene and chlorine starting products into the reaction system and in the use of atmospheric oxygen as a process adjuvant. In a number of methods the reaction gases are introduced directly into the reactor, and in others they are entirely or partially brought in contact with ethylene dichloride in mixers and dissolvers situated outside of the reactor, and then brought to the reaction.
In such techniques, the preparation of ethylene dichloride at elevated temperature and hence elevated pressure creates special problems due to the cell chlorine derived from a chlorine-alkali electrolysis and the gaseous impurities contained therein, of up to 10% (hydrogen, nitrogen, oxygen and carbon dioxide gases). Thus the chlorine gas must be compressed to a pressure between 5 and 20 bar, depending on the reaction temperature, and this is difficult and requires a great deal of energy.
Furthermore, the exhaust gases leaving the reaction system after the separation of the ethylene dichloride contain small amounts of hydrogen chloride and chlorine, which are usually neutralized with soda lye. The resultant mixture of residual soda lye, sodium chloride, sodium hypochlorite and sodium carbonates in water, has to be disposed of. The same need exists in the withdrawal of liquid ethylene dichloride from the reactor and the then necessary neutralization of the hydrogen chloride; in addition to the above-mentioned waste substances, a considerable amount of iron hydroxide sludge is produced if iron(III) chloride is used as catalyst in the decanting apparatus for separating the organic and aqueous phases. When the necessary periodical cleaning work is performed, and during the rest of the process, substance losses occur, and the catalyst losses must be constantly made up.
It is an object of the invention to avoid any chlorine gas compression and largely supress the formation of higher chlorinated products and chlorine containing distillation residues.